It has been recognized for many years that an electrical outlet can constitute a hazard under certain circumstances and as to certain individuals, notably children, but also adults of limited mental capacity. Because the conventional outlet normally has two or more energized, electrically conductive contact surfaces which are rather easily reached through openings in an insulating cover plate, insertion of a pin or other electrically conductive probe can result in serious shock.
The recognition of this and similar problems has resulted in numerous efforts to provide a safer outlet, and some of these efforts are illustrated in the following United States Patents:
U.S. Pat. No. 2,336,218, Blinn PA1 U.S. Pat. No. 2,500,474, Sperrazza PA1 U.S. Pat. No. 2,545,536, Von Holtz PA1 U.S. Pat. No. 2,770,786, Czyzewski PA1 U.S. Pat. No. 3,238,492, Houston PA1 U.S. Pat. No. 3,601,758, Davidson PA1 U.S. Pat. No. 3,775,726, Gress PA1 U.S. Pat. No. 3,909,566, Morrison et al PA1 U.S. Pat. No. 3,980,370, Gonzalez-Hernandez
A review of these reveals that some of the solutions arrived at are extremely complicated and, therefore, have not received wide acceptance. Others do not provide adequately safe or reliable solutions. As will be recognized, it is extremely important that the outlet must not only solve the problem but it must do so in a structure which can be manufactured at reasonable cost. Otherwise, the safer outlet simply will not be used.
One of the more interesting solutions is that shown in the patent to Sperrazza (U.S. Pat. No. 2,500,474) which employs a pair of switch contacts within the outlet housing and operators to close the switch contacts when the plug blades are inserted into the outlet. The concept employed in the Sperrazza structure is that the wire leading to each of the two female elements in the outlet is connected through a normally open series switch and the switch which supplies power to one female element is closed when a plug blade (or similar object) is inserted into the other female element. Thus, in order for power to be supplied to both female elements, blades must be inserted into both. As will be recognized, insertion of a foreign object into one female element presents no danger because that insertion only energizes the other element into which nothing has been inserted.
While this is clearly a valid concept insofar as safety is concerned, the structure of Sperrazza has certain disadvantages largely associated with normal usage of the outlet. First, it will be seen that the switch contacts are closed after the plug blades have made contact with the female elements, the purpose of this arrangement being to avoid arcing at the outlet front opening location. It has been found, however, that arcing occurs, under some conditions, across the switch contacts themselves. This arcing tends to degrade the contacts, shorting their lives and requiring replacement of these relatively expensive components. The switch contacts are necessarily rather small in surface area so that, under arcing conditions, the current density is high and the resulting arc relatively hot. Thus, the device has limited life.
Second, the Sperrazza device is limited in its flexibility in the sense that it is designed to receive only plug blades which are essentially parallel to each other. While this is suitable for some forms of plugs, there are other blade arrangements for which safe receptacles could and should be provided, and the Sperrazza structure does not appear to be usable with at least some other arrangements, notably orthogonal blade orientations, because of the disposition of switch components within the housing.
Third, it is common practice today, and is required under some conditions, to provide a third prong on the plug for grounding purposes and to provide a grounded female element in the receptacle to receive that prong. The Sperrazza structure has no provision for a grounding element and, again because of the limited space and the arrangement of parts necessary to allow the Sperrazza device to operate as intended, a place for a grounding element does not exist.
Fourth, Sperrazza employs relatively long and narrow curved spring elements which not only occupy much of the valuable space within the receptacle housing but also involve considerable heat generation. The spring elements are part of the current-carrying circuits in the receptacle and thus generate heat in accordance with the well-known power dissipation relationship I.sup.2 R. While the resistance of the elements would certainly be made as low as possible, there is a practical lower limit because the material used and the geometry must be chosen to provide the necessary spring characteristics. The result is some internal heating within the receptacle housing.